The oceanic gel phase: a bridge in the DOM–POM continuum

Recent discoveries reveal that polymer gel particles are abundant and important in the microbial loop, sedimentation processes, biogeochemical cycling, marine carbohydrate chemistry, and particle dynamics in the ocean. The novelty of these discoveries elicited an interdisciplinary discussion among investigators working in marine geochemistry, microbiology, and polymer physics on the significance of gels in the functioning of marine ecosystems. Marine gels are three-dimensional networks of biopolymers imbedded in seawater. They range in size from single macromolecules entwined, forming single-chain colloidal networks, to assembled polymer networks several hundreds of microns or larger. Gels can form in minutes to hours from dissolved organic matter or polymer chains released by phytoplankton or bacteria. They enclose nanoscale microenvironments that exhibit emerging physical, chemical, and biological properties that are drastically different from those of the DOM polymers that make them. Previous studies show that ~10% of surface DOM could be assembled as gels, yielding estimates of ~70� 10 15 g of organic carbon. This figure exceeds the global biomass of marine organisms by a factor of 50. The potential huge magnitude of the oceanic gel organic matter (GOM) pool suggests a need to develop reliable quantitative methods to systematically investigate the budget of marine gels and their role in biogeochemical cycling. Gels are particularly important for carbon cycling in that they provide an abiotic mechanism to move organic molecules up the particle size spectrum to sizes capable of sedimentation and eventual sequestration in the deep sea. Macrogels such as transparent exopolymer particles (TEP) are especially significant in sedimentation processes because they appear to be critical for the formation of marine snow and the aggregation of diatom blooms. The discovery of highly abundant gels in seawater also fundamentally changes how we think about the physical nature and microscale structure of the fluid and organic matter field encountered by bacteria, protists, and viruses in the sea. Gels may serve as nutrients and/or attachment surfaces for microbes, as refuges from predation, and as hot spots of high substrate concentration.